‘It seemed silly not to take a chance’ – Firth

“Every time I got a bruise or a broken tooth, rather than get the nurse in, it was, ‘Get the cameras in, so we can show you’re going through it!’” recalls the 54-year-old.

That’s because in his latest incarnation, the man known to millions as Mr Darcy, from both the Pride And Prejudice TV series and Bridget Jones movies, has turned action spy and martial arts expert, in the comic book adaptation Kingsman: The Secret Service.

“When we were shooting it, the stunt guy would say, ‘Let’s get the B-roll in to film some of this, because nobody’s going to believe it’.

“It was unexpected for me too, but it sparked my interest immediately because of that,” admits the Hampshire-born actor, who was already intrigued by the director Matthew Vaughn, who also helmed Kick-Ass and X-Men: First Class.

“I think he’s a one-off. There was no script yet, it was a work in progress and he wasn’t ready to show it to me, so I just went to hear what he had to say and I found him very compelling,” reveals Firth, who was Oscar nominated for Tom Ford’s A Single Man in 2010, and walked away with the gold statuette for The King’s Speech the following year.

“Something in me just went, ‘Why not?’ It’s so unusual for me, that it seemed silly not to take a chance.”

Early on, Vaughn was “full of dire warning” of the extent of training he’d be required to do. “And how much it would hurt and how much I was going to hate him by the end of it, and was I up for that,” Firth adds.

“It was three hours a day for about six months, and started off in my back garden with a squat, lunge and somersaults, and then the choreography kicked in. I was learning to use parts of my body that I’d never used – I didn’t even know they existed. It was painful, but I warmed to it and found it exhilarating,” he continues.

“This wasn’t the stuff I was doing when I was young. I wasn’t one of the more athletic drama students. My recreation was a lot more sedate in my youth, so it was a chance in my 50s to do something completely different, and go into a bit of a physical world for a while.”

Bar one stunt, involving a rig, it’s all Firth on screen. “I said that to somebody, and they looked at me with horrified disappointment that I would tell people such an egregious lie,” he notes, smiling.

“One of the reasons I feel so driven to tell people, is that I won’t have gone through all that pain for nothing.”

The effort definitely paid off. Not only did Livia, his Italian wife of 17 years, appreciate her newly honed husband (“It got noticed!” he confesses, laughing), but he looks a natural in the role of Harry, an impeccably suave spy and member of the secret organisation, known as Kingsman.

Harry recruits street kid Eggsy, played by newcomer Taron Egerton, and trains him up just as a global threat emerges from a twisted tech genius, brought to life by Samuel L. Jackson. A pastiche of the spy movies we all know and love, Firth acknowledges there’s a lot of violence - but “it’s wildly implausible”.

“It’s like a provocative pantomime, and in some ways I think it’s sending up the extremity. I can’t imagine how anyone would emulate it, because you’d need to be trained to do that stuff. It’s closer to what you get in Monty Python, than it is to the realistic films about the Middle East.”

The movie marks his third collaboration with Nicole Kidman, following The Railway Man and Before I Go Sleep. He grins when asked if they’ll work together again. “Inevitably,” Firth replies. “I think we’ll go for the multiple box-sets.”